Application of Photoluminescence

transcript

In Analyzing Optimal Growth Factors in Quantum Nanowires

I. Introduction

Despite the increasing predominance of solar energy in the search for alternative

energy sources, the uneconomical nature of solar energy has been hindering the energy

source from greater prevalence and popularity. Although the use of solar energy has

gradually been rising for several years, it is still widely criticized for its costliness and

inefficiency. Currently, many photovoltaic cells exploit planar semiconductors to conduct

energy for applicable use. Planar semiconductors utilize semiconductor materials, often

layered vertically on top of one another. This process requires intricate construction in

Molecular-Beam Epitaxy Labs as well as careful consideration of factors such as

substrate material and lattice structure of each element used to construct the

semiconductor. [5] In planar semiconductors, materials are arranged in a manner such

that the material of the top layer contains the greatest band gap, allowing it to absorb the

highest energy photons, leaving the layers beneath with smaller band gaps to absorb the

remaining photons. Materials play a vital key role in the efficiency of the semiconductor.

Materials such as amorphous silicon and organic materials offer an economical

alternative, but are greatly inefficient in obtaining energy. On the other hand, materials

such as Gallium Arsenide (GaAs) are often very costly but whose efficiency offer the

highest percentage of 28 percent in planar semiconductors. [21] The uneconomical and

impractical approach to producing planar semiconductors has been a major drawback in

the production of such use of semiconductors in devices such as solar cells; instead,

recently researchers have been turning to explore the advantages of utilizing nanowire

semiconductors to resolve the economics of solar energy.

Quantum Nanowires

Nanowires specialize in their ability to harvest great amounts of energy in a

relatively miniscule length-to-width ratio. Nanowires are often approximately 100

nanometers in length; because of the size of these structures, such technology often

ranges into the field of quantum mechanics. Given its material, nanowires have the ability

to act as insulators, semiconductors, and conductors.

Unlike planar semiconductors, nanowire semiconductors present a more

pragmatic alternative because the only the nanowires, which range from 1 to 4

micrometers in height, require materials such as Gallium Arsenide to achieve efficiency.

Figure 1: A Scanning Electron Microscope (SEM) images Gallium Arsenide nanowires grown on a

Silicon substrate. The transmission electron microscope (TEM) illustrates a single nanowire; the final

Scaning Transmission Electron Microscope (STEM) image displays the atomic structure of an

individual nanowire.

(Duan, Wang, Lieber, 2000) The substrate on which these nanowires are produced is

often made of more inexpensive materials, such as silicon dioxide (SiO2). (Dick, 2008)

Several benefits that nanowires provide include long absorption path lengths and short

distances for carrier charge transport; a strong ability to capture light; and alterations of

material properties and cell efficiencies through dimension and composition deviations of

the nanowires. (Yang, Yan, and Fardy, 2010)

The three-dimensional structure of nanowires allows more photons to be captured

than seen with planar two-dimensional structures. The size of these nanowires, in

addition to its potential efficiency in capturing light and converting the light to usable

energy, decreases the cost per watt of devices utilizing such structures.

Unfortunately, the current obstacle inhibiting the success of nanowires is the low

efficiency level of 6 percent primarily due to it being relatively new idea in comparison

to planar semiconductors. Nevertheless, nanowire semiconducors present a promising

future for solar energy as research advances.

Semiconductor Applications in Solar Cells

When a source of light strikes a photovoltaic cell, the semiconductor absorbs the

photons. The energy of the photons is able to knock loose electrons in the semiconductor;

thus allowing the electrons, know as carrier charges, to “jump” in the conduction band.

Semiconductors use a process known as “doping” to increase the efficiency of generating

current. The process of doping involves adding impurities to a material, most often

Silicon. [1] In GaAs semiconductors, N-type doping involves adding a minute quantity of

Arsenic, which contains five electrons in its valence shell, also known as valence band.

Because Silicon only contains four outer electrons, the fifth electron from the Arsenic is

very loosely bound. As a result, relatively small amounts of energy, such as the energy

from photons, are able to “knock loose” these electrons into the conduction band,

creating a flow of current. The conduction band is the range of electron energies in which

electrons are delocalized and are able to conduct electricity.

In addition to N-type doping, P-type doping involves adding Gallium, which

contains only three outer electrons. When these three outer electrons bond to silicon, a

fourth electron from Silicon is unable to bind with another electron from Gallium, this

creating a “hole” where the electron is absent. The absence of an electron creates a

positive charge, which is also able to conduct a current in the semiconductor.

Photoluminescence

Figure 2: Semiconductors possess a relatively small band gap between the

valence band and the conduction band. The band gap represents the amount of

energy required to “knock loose” an electron from its valence shell. In insulators,

the size of the band gap is much greater; therefore, electrons are not easily

delocalized, resulting in the nonconductive nature of insulators. On the other

hand, the conduction band and valence band in metal conductors often overlap,

resulting in the conductive nature of conductors.

The process of photoluminescence is utilized to characterize semiconductors.

Photoluminescence utilizes Einstein’s Photoelectric Effect, in which photons and

maximum kinetic energy are released as the energy of a beam of light surmounts the band

gap of the semiconductor. In the process of photo-excitation, electrons “jump” into their

excited states; as the electrons assume back into their ground state, excess energy is

emitted in the form of photons. The amount of energy in emitted light from the sample,

known as photoluminescence, can also be used to measure the band gap of new

compound semiconductors for characterization.

The purpose of this study is to observe the photoluminescence of diverse GaAs

quantum wire samples grown in Molecule Bean Exitaxy (MBE) labs under various

conditions, including temperature, etc. The photoluminescence of each sample measures

the luminosity or amount of photons that are emitted from the nanowire sample as an

incident light source, such as a Helium-Neon laser or a white light source, strikes the

sample. In addition to measuring the efficiency of the sample, photoluminescence is also

imperative in characterization of semiconductors and recognition of contamination often

found during its epitaxial growth stages. This measurement is directly related to the

efficiency of the nanowire sample; as the amount of photons yielded from the sample

increases, the greater the effectiveness of a particular sample.

Figure 3: Einstein’s Photoelectric

Effect demonstrates the release of

photons as electrons fall back to their

ground state after a process of photo-

excitation occurs.

II. Methodology

Growth and Fabrication of Nanowire Semiconductors

There are two methods to fabricate nanowires: “top-down” and “botton-up”. The

top-down technique involves carving down a bulk of the desired material to the desired

size. Although this process had been more widely used in the past several decades,

problems arise when technology begins to demand smaller and smaller structures.

The “bottom-up” technique, often associated with epitaxial growth, is the more

prevalent method used for nanowire growth today and also utilized for the growth of

GaAs samples in this experiment.[25] Epitaxial growth involves the oriented growth of

crystalline structures, usually grown on a crystal substrate. This technique allows for a

controlled chemical composition in addition to the ability to easily fabricate smaller

structures unlike the “top-down” technique.

In the experiment, Molecular Beam Epitaxy (MBE) was used to grow the

semiconductor nanowire samples. MBE requires a high vacuum environment, in which a

low deposition rate of elemental beams of material occurs. [25] First, Gallium and

Arsenide are heated to a sublimation temperature to become gaseous atoms. During this

stage, the two materials remain in separate gaseous chambers; the term “beam” signifies

that the two materials do not interact with one another until both materials reach the

wafer. At this point, the two materials will condense on the Si wafer to form a single

crystal using quantum wells to direct the growth of GaAs. [22]

In addition to the Gallium and Arsenide, an additional elemental particle is

essential in the growth of the nanowires. [5] In this particular experiment, gold

nanoparticles were applied to promote the growth of the nanowire in one dimension. Au

particles are currently the most commonly used materials, largely due to the extensive

research concentrated on this material.[24]

A problem that arises with the use of Au particles is the rise of contamination in

the Silicon wafer. [5] As Au particles hit the surface of the wafer, often the particles will

submerge itself into the bandgap of the Si wafer, thus negatively affecting the electrical

Figure 4: Molecular Beams of Gallium

and Arsenide coat the Si wafer in this

“bottom-up” technique

Figure 5: SEM photograph of GaAs

nanowires grown in MBE lab used in this

experiment.

conductivity of the semiconductor. As a result, small quantities of Au particles must be

applied at a time in order to minimize the diffusion of Au into the Silicon surfaces.

Procedure

In determining the photoluminescence of the given GaAs nanowires samples, an

optical set-up was required to direct the incident and photoluminescent light in the

following path.

The original source of light is emitted from a helium-neon laser or a white light

source through a series of mirrors and lenses to the optical chopper, which modulates the

intensity of the incident light.[19]

Next, the light was directed to the cryostat containing the nanowire semiconductor

samples, which absorb the incident light. The cryostat is responsible for lowering the

Figure 5: Conceptual diagram of optical photoluminescence set-up; beams

of light originating from either the He-Ne laser or white light box follow

the pattern of optics to the cryostat, where the semiconductor nanowires

are contained.

temperature of the nanowire samples to increase emitted photons. The lower temperature

will increase the intensity of emitted light and reduce random scattering of electrons and

holes.

Then, the photons released from the nanowire samples were then directed to a

monochrometer and photomultiplier tube. The monochrometer is responsible for

narrowing the range of light to a select wavelength.[16] Following the monochrometer,

the photomultiplier multiplies the current of the wavelength selected by the

monochrometer by Einstein’s Photoelectric Effect and Secondary Emission.

Next, the current was delivered to the lock-in amplifier, which modified the signal

to reduce obstructive noise from the surrounding environment. The lock-in amplifier

multiplies the input reference signal (ωr), the signal from the optical chopper, by the input

signal (ωs), the current from the photomultiplier tube, to generate two Alternating Current

waves (ωr+ωs and ωr-ωs). These two waves then pass through a low-pass filter, which

allows low frequency waves to bypass while removing out any frequencies higher than

the set cutoff frequency. The low-pass filter generally eliminates the two Alternating

Current s unless the ωr and ωs are equivalent, which results in a Direct Current that can

generate a voltage and is proportional to the signal amplitude.

Figure 6: conceptual diagram of interactions between reference signal and input signal; in the case where both signals are equal, a direct current is produced

Finally, the signals and data recorded by the various instruments are delivered to a

Labview program for data analysis.

III. Results and Discussion

The results of this experiment are still pending due to technical

miscommunications between laboratory instruments. Although data is currently

inconceivable, this setback does not hamper the significance of the project.

By analyzing the photoluminescence of each sample, we will be able to determine

which growth conditions will help yield the greatest amount of photons from the

semiconductor. In order to enhance the quality and efficiency of nanowire semiconductor

technology, it is essential to cultivate the process of producing an efficient

semiconductor. By combining the efforts of maximizing nanowire efficiency during its

growth stages with the work of various other labs to maximize efficiency in other areas of

the process, the effectiveness of nanowire semiconductors will begin to grow vastly.

The following photoluminescence samples illustrate the potential results of this

project. By comparing the various graphs in regards to growth conditions, rather than

Helium-Neon laser excitation power as displayed in the examples below, it becomes

possible to distinguish which growth condition yields the greatest count. The count on the

y-axis reflects the number of photons emitted from the semiconductor after the light

source strikes the cryostat. As figures 7 and 8 below display, the count can differ

immensely from one particular test to another. By garnering the greatest number of

counts possible, it is directly reflecting the semiconductor’s ability to produce an efficient

amount of electricity.

In addition to obtaining the count of each GaAs semiconductor sample,

photoluminescence can also display any contaminations in the semiconductor during

growth procedures. Each visually significant peak apart from the middle peak represents

contaminations in the semiconductor. As previously mentioned, the Au particles that had

dissolved into the Silicon wafer during Molecular Beam Epitaxy display its negative

effects on the quality of the semiconductor in photoluminescence diagrams. [22]

IV. Conclusions

Despite promising future that nanowire semiconductors hold for photovoltaic

cells, the low efficiency of nanowires is nonetheless a major topic of research. In this

project, the photoluminescence measurements of each semiconductor sample, each

labeled with and grown under different conditions in the MBE lab, will suggest which

Figures 7 and 8: results of photoluminescence in a GaAs semiconductor

sample testing the effects of power of the light source. The graphs display

different counts of emitted photons as well as reveal contamination of the

Silicon substrate during epitaxial growth.

growth conditions will benefit the efficiency and exploitation of photovoltaic’s using

nanowires. Apart from analyzing ideal growth conditions to optimize efficiency of

nanowire semiconductors, a relevant amount of research has also been conducted to

search for alternatives to optimizing semiconductors.

Many researchers continue to demonstrate an interest in increasing the efficiency

from solar power to electricity. Through the method of direct water electrolysis using p-n

junction doping, researchers have designed an effective technique to increase the

production of hydrogen.[14,20] Such materials have also proved to be effective in the

process of passivation, the coating of the junctions to preserve the condition of the cell. In

addision, using Indium Gallium Arsenside (InGaAs) for passivation not only provides

protection from environmental stresses on the cell, but has also increased the

effectiveness of power conversion.[21] By various arrays of design implementations of

Group III-V semiconductors, the collaborations of such research can ultimately lead to a

solution to the relative inefficiency in the power conversion of semiconductors in tandem

cells.

The success of GaAs nanowires are far from optimal, as demonstrated by the

ongoing research pertaining to such materials. In order to obtain greater success with

nanowires in solar cells, it is important to characterize the nanowire through

photoluminescence, which utilizes the photoelectric effect in displaying the bright light

spectrum emitted by the semiconductor when it is struck by light, such as a white light

box or a laser. After it is characterized, this information would facilitate the

understanding of how to construct a more efficient solar cell using the information

gathered from the photoluminescence. Research pertaining to the use of GaAs nanowires

to power solar cells will further augment our abilities to fabricate a photovoltaic of

greater efficiency.

By observing the optical properties that improve the functionality such nanowires,

further research would be able to enhance the efficiency of devices that utilize

semiconductor nanowires. Apart from analyzing optimal growth factors, it is also

essential to comprehend other factors that would contribute to the progression of

nanowire semiconductors. Photovoltaics, which are often associated with

semiconductors, are also a major topic of study as researchers are searching for

techniques to enhance such devices.[9,10] The results of this project, as well as similar

research on the topic of cultivating the efficiency of nanowire semiconductors, will be

able to enhance the overall effectiveness of solar cells. By implementing the use of

semiconductor nanowires in photovoltaics in the future when the efficiency of nanowires

surmount that of planar semiconductors, the cost-friendly approach to solar energy could

potentially kindle a newfound interest in solar energy.

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Application of Photoluminescence

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